Today in 1979 the so-called "Vela Incident" took place. What was it?

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Two mysterious flashes of light in the atmosphere above the southern Indian Ocean. The American Vela Hotel satellite, designed to detect nuclear explosions, registered the flashes as occuring near the Prince Edward Islands (above, in red) in the sub-Antarctic Indian Ocean. Theories as to what caused the flashes include: the satellite was struck by orbiting microdebris; a rare occurence of "superbolt" lighting; and a secret nuclear test by the governments of South Africa and Israel. To date there has been no conclusive explanation.